Incorrect mpg

[RussTaylor]

Oh man, we're going to go around and around in circles.

"You drove 110 miles total and used 50L plus some unknown amount, it was more than 10L because you stopped at 10L. You have used 50L + X, the unknown amount"

It doesn't matter, you know how much it takes to return it to full, therefore you know how much you used. You drove and used it, and you filled it back up, you know how much you used by how much it took to fill up.

X + Y + Z is the sum of all the partial fill ups + the amount required to return it to full.

So we have a huge coke bottle (say 75ltrs), and it took an amount (doesn't matter) to fill it up. We now have a full coke bottle, this is our reference.

You drink some, a bit more, you don't know how much but to help this discussion we'll say 10ltrs. So you have 40ltrs in there, you don't know that. You do the same two more times, you use another 20ltrs and now you're down to 30ltrs. But you don't know any of these measurements, because it doesn't matter.

So you have your full - x - y - z.

You then put 5ltrs in.

You drive around some more, so Full - x - y - z + 5ltrs - a.

You drive some more, then partial fill up (10 ltrs) then drive some more. So Full - x - y - z + 5 - a - b + 10 - c.

By now you've no idea what you have, apart from whatever remains of your original tank + 15 ltrs.

You fill it up (returning to our reference), this tanks 55ltrs.

You now know x + y + z + a + b + c = 55ltrs + 15ltrs = 70ltrs.

I have a way of knowing, because I have TWO references, both times the tank was FULL.

I will know how much coke I have drunk as soon as I fill the coke bottle right to the top. The amount I drank is 10L + 20L + 30L + whatever it takes to return it to the top.

[Winky]

Sorry Ciderbarrel, but I have to agree with RussTaylor on this one. In his example he wrote "...and perform a **FULL** fill up..." (emphasis added). In your rewriting of his example, this time with the giant coke bottle, you wrote "...and put 30L back in, it is still **NOT** full." (emphasis added) It looks like you weren't trying to deceive but just didn't read RussTaylor's example correctly.

In the examples above you don't have to know what the individual numbers are for x, y, and z, you just have to know that x + y + z = 60L.

I would say don't worry about adding partial fill-ups just aggregate the amounts on your own. Sure, you might go 800mi and add 80L to a 60L tank (or some other made up numbers) but you would get an accurate mileage for that time frame. This would really mess up trending though so I almost never do it even before I started using this site.

[Ciderbarrel]

Oh crap, yeah I totally missed that last full fill up in his post. That changes EVERYTHING. Ignore my last post because that is way off since I missed that last fill statement. I'm sorry I missed that.

Russ's example still doesn't jive because a 10L partial and a 20L partial won't equal a 30L because of the used fuel between 1st full tank and the last full tank.

The 50L for the first tank and the 30L for the last are valid. 10L of fuel was used because that 1st partial will ALWAYS be less than what was actually used, so 90L out of some X number of fuel was used.

Some more driving was done and some more fuel was burned and 20L was put back into the tank. Was 20L of fuel actually used? That is the point I'm trying to make. A partial after a partial doesn't tell you how much fuel was used, only bought. It isn't going to be as simple as Russ's example where his math somehow works out where his next full tank EXACTLY adds up to the last two partials.

Simple logic will tell you that putting in 10L and not hitting full and then 20L more and not hitting full means that the next full tank will take MORE than 30L because between the two partial fills he used MORE than 30L.

I hope I'm not over complicating things but here is another stupid example from me: a 100L tank (to keep the math simple.

Tank was full after the 50L fill, so there is 100L in it.

Some driving done, 10L put back in, not full. For arguments sake, lets say it has 99L in it. That would be a best case scenario. 10L bought, 11L used

Some more driving done, 20L put back into it. Again, let us assume that it is back up to 99L. 30L bought, 32L used.

Buying 30L can not get it back up to 100L. That is my point.

Also, if we want to avoid all this guessing, just top off your tank. I know in the UK the gas is way more expensive than here in the States, but it is the best way to be as accurate as possible.

[RussTaylor]

Ok I'm glad we're getting somewhere.

Fuel prices are different everywhere, I'll stick £10 in before I go home tonight and then do a full tank then (£80). Also as this is a website about fuel economies you can increase your economy by not driving around with a full tank. I think this is a fairly key point for this site.

Anyway Cider your new example isn't right either, sorry

"Tank was full after the 50L fill, so there is 100L in it.

Some driving done, 10L put back in, not full. For arguments sake, lets say it has 99L in it. That would be a best case scenario. 10L bought, 11L used

Some more driving done, 20L put back into it. Again, let us assume that it is back up to 99L. 30L bought, 32L used.

Buying 30L can not get it back up to 100L. That is my point."

No, but you do not have a full tank. So do a FULL tank fill up at the end and tally it all up

You don't specify how much you used in the second bit of driving, but as you're now up to 99ltr having put 20ltr back in, it must have been down to 79ltr. So you used 20ltr. So 30ltr of fill ups, you've used 31ltr, you don't know how much is in the tank but you said it was 99ltr. So top it up to a FULL tank (our reference) = 1 extra litre. So 31ltrs.

Basically there is no magic, whatever you put in is either in the tank still or has been used. I suppose simply put

If I have a full tank, drive 100 miles, and put 50 litres of fuel in to get back to full

is that any different to

I have a full tank, drive 50 miles, put in 25 litres, drive fifty miles, put 25 litres of fuel in

Assuming both examples return the same economy either way I've driven 100 miles and consumed 50ltrs. One is one full fill up, one is one partial fill up followed by a full fill up.

Anyway sorry to write so much and it gets very wordy, back to my original point I'd love partial fill ups to count as most of the time people here don't do full fill ups. If we did it once in a while to get an average that'd would suit us well

Thanks again

[MMUK]

Once you have a LOT of tanks in the system, the running average will be reasonably accurate regardless of whether there are partials present or not (since the 50 litres error would become less significant overall). It's the individual tank MPGs which would be missing.

Personally I never do partials. I know my range and which are the cheaper stations in the areas I normally drive.

[RussTaylor]

I climb 1200 feet to get to work each day, so if I run a half tank I save myself 50kg in weight dragging it up the hill, and seems to make a difference I'd only ever do part tanks if I had the choice.